CHEMOTAXIS 27 
tion of the body, no seeking of the stimulus and no attraction 
in any sense. The Parameecia as they happen to approach 
a drop of dilute chemical to which they are positively 
chemotactic, swim into it without interruption, but when 
they encounter the other side of the drop and experience 
the transition from weak acid to water they give the motor 
reflex or avoiding reaction and change their course. As 
often as the Paramcecium encounters this boundary it 
reacts in the same way. It behaves as if caught in a trap 
which presents no obstacle to its entrance, but effectually 
prevents its exit. Other Parameecia swimming by chance 
into the drop are also caught there and soon a collection is 
formed which may ultimately include all the individuals of 
the region. In the negative response the method is much 
the same only it is the transition from weak to strong acid 
which causes the stimulation. The avoiding reaction is 
given in the same way in each case. All of the chemotactic 
responses take place without any orientation of the body, 
by a method of trial and error. 
In the flagellate Chilomonas Jennings has shown that. 
chemotactic grouping takes place just as in Paramecium. 
In the bacterium Spirillum he finds much the same method 
employed. Spirillum swims by the movement of flagella 
at one or both ends of the body. When stimulated it 
simply reverses its direction of movement. Positive and 
negative reactions take place in exactly the same way. 
What things are avoided and what are sought depends 
upon what acts as a stimulus causing the reversing reaction. 
Apparently these simple organisms have no power of 
orienting their bodies; they simply move back and forth, 
relying on chance to get them into a favorable situation. 
When such a situation has been reached the organisms 
remain relatively quiet. In several cases among the flagel- 
